Branch — pointer, HEAD, switch আর detached HEAD

$ git branch feature/payment
$ git branch
  feature/payment
* main
$ git branch -v
  feature/payment ee98167 Add cart page
* main            ee98167 Add cart page
$ cat .git/HEAD
ref: refs/heads/main
$ cat .git/refs/heads/main
ee981670c64fd4824100b660959c4be958cb034f
$ cat .git/refs/heads/feature/payment
ee981670c64fd4824100b660959c4be958cb034f

// A tiny model of Git refs. Commits form a graph; a branch is only a name
// pointing at one commit (like .git/refs/heads/<name>); HEAD names the branch you are on.
const commits = {};              // id -> { parent }
const branches = {};             // name -> commit id
let HEAD = { ref: "main" };      // attached: { ref }, detached: { id }
let n = 0;
const headId = () => (HEAD.ref ? branches[HEAD.ref] : HEAD.id);
function commit() {
  const id = "C" + ++n;
  commits[id] = { parent: headId() || null };
  if (HEAD.ref) branches[HEAD.ref] = id;   // only the CURRENT branch moves
  else HEAD.id = id;
}
const branch = (name) => { branches[name] = headId(); };  // new pointer, no new commit
const sw = (name) => { HEAD = { ref: name }; };
function history(id) {                      // follow parent links back to the root
  const out = [];
  for (; id; id = commits[id].parent) out.unshift(id);
  return out.join("--");
}
const run = [
  ["git commit  (C1)", commit], ["git commit  (C2)", commit],
  ["git branch feature", () => branch("feature")],
  ["git switch feature", () => sw("feature")],
  ["git commit  (C3)", commit], ["git commit  (C4)", commit],
  ["git switch main", () => sw("main")],
  ["git commit  (C5)", commit],
  ["git switch -c fix", () => { branch("fix"); sw("fix"); }],
];
for (const [cmd, act] of run) {
  act();
  console.log("$ " + cmd);
  for (const [name, id] of Object.entries(branches))
    console.log("   " + name.padEnd(8) + history(id) + (HEAD.ref === name ? "   <- HEAD" : ""));
}

$ git switch feature/payment
Switched to branch 'feature/payment'
$ git commit -m "Add payment form"
[feature/payment 9c3478e] Add payment form
 1 file changed, 1 insertion(+)
 create mode 100644 payment.txt
$ git commit -m "Validate phone number"
[feature/payment fc2d7cf] Validate phone number
 1 file changed, 1 insertion(+)
$ git branch -v
* feature/payment fc2d7cf Validate phone number
  main            ee98167 Add cart page

$ git switch main
Switched to branch 'main'
$ git switch -c fix/cart-total
Switched to a new branch 'fix/cart-total'
$ git commit -m "Fix cart total rounding"
[fix/cart-total c78d7eb] Fix cart total rounding
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git switch main
Switched to branch 'main'
$ git merge --ff-only fix/cart-total
Updating ee98167..c78d7eb
Fast-forward
 cart.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git log --oneline --graph --all
* c78d7eb (HEAD -> main, fix/cart-total) Fix cart total rounding
| * fc2d7cf (feature/payment) Validate phone number
| * 9c3478e Add payment form
|/
* ee98167 Add cart page
* 1195e12 Add product list

$ git switch ee98167
fatal: a branch is expected, got commit 'ee98167'
hint: If you want to detach HEAD at the commit, try again with the --detach option.

$ git branch --merged
  fix/cart-total
* main
$ git branch --no-merged
  feature/payment
$ git branch -a
  feature/mobile-payment
  feature/payment2
* main
  remotes/origin/main

$ git status --short
 M README.md
$ git switch feature/payment
Switched to branch 'feature/payment'
M	README.md
$ git status --short
 M README.md

$ git status --short
 M cart.txt
$ git switch feature/payment
error: Your local changes to the following files would be overwritten by checkout:
	cart.txt
Please commit your changes or stash them before you switch branches.
Aborting
$ git branch --show-current
main

$ git checkout ee98167
Note: switching to 'ee98167'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at ee98167 Add cart page
$ cat .git/HEAD
ee981670c64fd4824100b660959c4be958cb034f
$ git branch
* (HEAD detached at ee98167)
  feature/payment
  fix/cart-total
  main

$ git commit -m "Try grid layout"
[detached HEAD 2cbf7b9] Try grid layout
 1 file changed, 1 insertion(+)
 create mode 100644 layout.txt
$ git switch main
Warning: you are leaving 1 commit behind, not connected to
any of your branches:

  2cbf7b9 Try grid layout

If you want to keep it by creating a new branch, this may be a good time
to do so with:

 git branch <new-branch-name> 2cbf7b9

Switched to branch 'main'
$ git reflog -5
c78d7eb (HEAD -> main, fix/cart-total) HEAD@{0}: checkout: moving from 2cbf7b91df472dd80f0b9e90329a8f0977a75b66 to main
2cbf7b9 HEAD@{1}: commit: Try grid layout
ee98167 HEAD@{2}: checkout: moving from main to ee98167
c78d7eb (HEAD -> main, fix/cart-total) HEAD@{3}: checkout: moving from feature/payment to main
fc2d7cf (feature/payment) HEAD@{4}: checkout: moving from main to feature/payment
$ git branch experiment/grid 2cbf7b9
$ git log --oneline --graph --all
* 2cbf7b9 (experiment/grid) Try grid layout
| * c78d7eb (HEAD -> main, fix/cart-total) Fix cart total rounding
|/
| * fc2d7cf (feature/payment) Validate phone number
| * 9c3478e Add payment form
|/
* ee98167 Add cart page
* 1195e12 Add product list

$ git switch --detach main~1
HEAD is now at ee98167 Add cart page
$ git status
HEAD detached at ee98167
nothing to commit, working tree clean
$ git switch -c hotfix/old-cart
Switched to a new branch 'hotfix/old-cart'

// Detached HEAD: commit, walk away, find the commit again through the reflog.
// Ids come from FNV-1a 32-bit, a small stand-in for Git's SHA-1 (not secure).
function fnv1a(s) {
  let h = 0x811c9dc5;
  for (const ch of s) { h ^= ch.charCodeAt(0); h = Math.imul(h, 0x01000193) >>> 0; }
  return h.toString(16).padStart(8, "0").slice(0, 7);
}
const commits = {}, branches = {}, reflog = []; let HEAD = { ref: "main" };
const tip = () => (HEAD.ref ? branches[HEAD.ref] : HEAD.id);
function commit(msg) {
  const id = fnv1a((tip() || "") + "|" + msg);
  commits[id] = { parent: tip() || null };
  if (HEAD.ref) branches[HEAD.ref] = id; else HEAD.id = id;
  reflog.unshift({ id, why: "commit: " + msg });
}
function move(to, detached) {
  const from = HEAD.ref || HEAD.id;
  HEAD = detached ? { id: to } : { ref: to };
  reflog.unshift({ id: tip(), why: "checkout: moving from " + from + " to " + to });
}
function orphans() {                       // commits no branch can reach any more
  const seen = new Set();
  for (const t of Object.values(branches))
    for (let id = t; id && !seen.has(id); id = commits[id].parent) seen.add(id);
  return Object.keys(commits).filter((id) => !seen.has(id));
}
commit("Add product list"); commit("Add cart page"); commit("Fix cart total");
move(commits[branches.main].parent, true);  // git checkout <main~1>: detached
commit("Try grid layout");
move("main", false);                        // git switch main
console.log("commits:", Object.keys(commits).join(" "), "| orphaned:", orphans().join(" "));
reflog.slice(0, 3).forEach((e, i) => console.log(`${e.id} HEAD@{${i}}: ${e.why}`));
const found = reflog.find((e) => e.why === "commit: Try grid layout").id;
branches["experiment/grid"] = found;        // git branch experiment/grid <id>
console.log("rescued", found, "| orphaned now:", orphans().length);

$ git branch -d fix/cart-total
Deleted branch fix/cart-total (was c78d7eb).
$ git branch -d experiment/grid
error: the branch 'experiment/grid' is not fully merged
hint: If you are sure you want to delete it, run 'git branch -D experiment/grid'
hint: Disable this message with "git config set advice.forceDeleteBranch false"
$ git branch -D experiment/grid
Deleted branch experiment/grid (was 2cbf7b9).
$ git branch -d feature/payment
error: the branch 'feature/payment' is not fully merged
hint: If you are sure you want to delete it, run 'git branch -D feature/payment'
hint: Disable this message with "git config set advice.forceDeleteBranch false"
$ git branch -m feature/payment feature/mobile-payment
$ git branch -v
  feature/mobile-payment fc2d7cf Validate phone number
* main                   c78d7eb Fix cart total rounding
$ git branch -d main
error: cannot delete branch 'main' used by worktree at '…/repo'

// git branch --merged / --no-merged as a reachability test on the commit graph.
// A branch is "merged into T" when its tip can be reached from T's tip by
// following parent links (a merge commit has two parents).
const parents = {
  A: [], B: ["A"], C: ["B"], D: ["C"],        // main line
  E: ["B"], F: ["E"],                         // feature/login, later merged
  M: ["D", "F"],                              // merge commit on main (two parents)
  G: ["M"],                                   // main moves on
  H: ["C"], I: ["H"],                         // feature/payment, never merged
};
const tips = { main: "G", "feature/login": "F", "feature/payment": "I", "fix/typo": "D", "old-spike": "B" };

function ancestors(start) {                   // all commits reachable from start
  const seen = new Set(), stack = [start];
  while (stack.length) {
    const c = stack.pop();
    if (seen.has(c)) continue;
    seen.add(c);
    stack.push(...parents[c]);
  }
  return seen;
}
function report(target) {
  const reach = ancestors(tips[target]);
  const merged = [], notMerged = [];
  for (const [name, tip] of Object.entries(tips))
    (reach.has(tip) ? merged : notMerged).push(name);
  console.log(`into ${target} (${reach.size} reachable: ${[...reach].sort().join("")})`);
  console.log("  --merged:    " + merged.join(", "));
  console.log("  --no-merged: " + notMerged.join(", "));
}
report("main");
report("feature/payment");

$ git check-ref-format --branch feature/login
feature/login
$ git check-ref-format --branch "feature/new login"
fatal: 'feature/new login' is not a valid branch name
$ git check-ref-format --branch "fix..cart"
fatal: 'fix..cart' is not a valid branch name
$ git check-ref-format --branch "release/v1.0.lock"
fatal: 'release/v1.0.lock' is not a valid branch name
$ git branch "x~1"
fatal: 'x~1' is not a valid branch name
hint: See 'git help check-ref-format'
hint: Disable this message with "git config set advice.refSyntax false"
$ git branch feature
fatal: cannot lock ref 'refs/heads/feature': 'refs/heads/feature/mobile-payment' exists; cannot create 'refs/heads/feature'